Depending on the use for which they are intended, parts made on the basis of aluminum alloy or of magnesium alloy must undergo operations of surface treatment, in order to increase their corrosion resistance. This applies in particular to parts intended for use in the aeronautical industry, which must meet stringent requirements, notably in terms of performance in the salt spray test.
There are at present several techniques for surface treatment of parts made of aluminum alloy, allowing their properties of corrosion resistance to be increased. One of these techniques is the treatment of chemical conversion of aluminum alloy. The commonest of these methods, known by its trade name “Alodine® 1200” of the company Henkel, performs a chromatizing treatment. For this purpose it employs a substance based on hexavalent chromium. This method is able to endow aluminum alloy with good corrosion resistance, while ensuring that the part has the ability to conduct electricity, by forming a complex surface layer composed principally of hydroxides and oxyhydroxides of chromium and of aluminum on the part, but it gives rise to an environmental problem. Substances based on hexavalent chromium have in fact proved to be toxic to living organisms.
Another technique used conventionally for significantly improving the corrosion resistance of parts made of aluminum alloy implements an anodizing step, followed by one or more sealing steps, i.e. of blocking or closing-up the pores present in the porous anodic layer created on the surface of the part by the anodizing step. There are several types of this. The commonest, employed to obtain a large increase in the corrosion resistance of the parts, notably in order to meet the requirements of the aeronautical sector, consists of chromic anodic oxidation, followed by hydrothermal sealing based on potassium dichromate. Once again, in these various steps, this method thus employs a substance based on hexavalent chromium, which is dangerous to health.
Regarding parts made of magnesium alloy, there are also at present several techniques of surface treatment for increasing their properties of corrosion resistance. One of these techniques is the treatment of chemical conversion of magnesium alloy. The commonest of these methods, known by the designation mordanting, carries out a chromatizing treatment. For this purpose it employs a substance based on hexavalent chromium. This method makes it possible to endow magnesium alloy with good corrosion resistance, by forming a complex surface layer composed principally of hydroxides and oxyhydroxides of chromium and of magnesium on the part, but it too gives rise to an environmental problem, for the same reasons as were expressed above.
Moreover, methods have been proposed in the prior art, notably illustrated by the documents U.S. Pat. No. 5,304,257, U.S. Pat. No. 5,374,347 or else WO 2006/088519, for surface treatment of parts made of aluminum comprising immersing the part in two successive baths, i.e. a first bath containing a metal-salt corrosion inhibitor other than a hexavalent chromium salt, and an oxidizing compound, and a second bath containing an oxidizing compound. The corrosion resistance of the parts thus treated does not, however, prove satisfactory, and it is notably less than that obtained by the treatments using hexavalent chromium.